(1) Field of the Invention
The present invention relates to a method for producing a modified conjugated diene polymer and a rubber composition using the modified conjugated diene polymer obtained thereby. More particularly, the present invention relates to a method for producing a modified conjugated diene polymer enhanced in low heat buildup (low fuel consumption) and reinforcing ability with a filler and excellent in quality stability, and a rubber composition using the modified conjugated diene polymer obtained thereby.
(2) Description of the Related Art
In relation to recent social requests for energy saving, requirements for low fuel consumption of automobiles are becoming severer. In order to comply with such requirements, it has been required that the rolling resistance of tires is further decreased also in tire performance. As techniques for decreasing the rolling resistance of tires, there have also been studied techniques of optimizing tire structures. However, it has been employed as the most general technique that materials lower in heat buildup are used as rubber compositions.
In order to obtain such rubber compositions having low heat buildup, there have hitherto been performed many technical developments of modified rubbers for rubber compositions using silica or carbon black as a filler. Of these, methods of modifying polymerization active ends of conjugated diene polymers obtained by anionic polymerization using organic lithium compounds with alkoxylsilane derivatives having functional groups which interact with the filler has been proposed as effective methods.
However, many of them are applied to polymers in which the living properties of the polymer ends can be easily secured, and a few are applied to the improvement by modification of cis-1,4-polybutadiene particularly important in tire side wall rubber and tire tread rubber. Further, the effect of modifying the rubber compositions into which silica or carbon black have been incorporated is not necessarily sufficient. In particularly, for cis-1,4-polybutadiene, the effect of modifying the rubber into which carbon black has been incorporated is scarcely obtained in the actual situation.
Further, many of the conventional modification techniques can not sufficiently impart branches to main chains, so that cold flow becomes a serious obstacle in practical application. Partial coupling to comply therewith necessarily poses the problem of decreasing the modification effect.
On the other hand, there is also an attempt to allow an active end of a conjugated diene polymer having a high cis content obtained by use of a rare earth catalyst to react with a functional group-containing alkoxysilane derivative which interacts with a filler, thereby obtaining an end-modifyied conjugated diene polymer. According to this method, the cold flow is effectively improved, but the Mooney viscosity is largely increased with time due to alkoxysilane modification. Accordingly, there has been still room for improvement from the viewpoint of quality stability.